Phone: Display toast message for blank USSD request

As per spec 24.080, ussd response string should be
between 1 - 160.
When user requests SEND_USSD with blank/no
input, display toast message prompting user
to enter valid input.

Bug: 7471891
Change-Id: Ic7b7b89e8a683011048a19cabd1d36a27bbae953